In conventional mammography, a woman places her breast on a breast support plate. A detector is typically mounted under the breast support plate. This detector is sensitive to x-rays. A breast compressor plate that is transparent to light and x-rays presses against the top of the breast to flatten it and to prevent any movement during the mammography process. An x-ray source is then turned on to image the breast, which is between the breast support plate and the breast compression plate.
The transmitted x-ray intensity through the breast is dependent on both the composition of the breast and its local thickness. Most current mammography machines have thickness indicators, which are imprecise. These thickness indicators do not show true breast thickness if either the breast support plate or the compression plate bend or flex. This, in turn, will affect the accuracy and precision of the images of the breast obtained.
Techniques for determining the degree of flex of the breast compression plate have been devised. For example, see Burch, A. and Law, J., A Method for Estimating Compressed Breast Thickness During Mammography. Br J Radiol 68, (1995) 394-399, which discloses using the magnification of lead markers placed on top of the compression plate. However, this method requires that all the markers be shown in the image, and does not measure the flexing of plastic plates.
Accurate measurement of compressed breast thickness is an important factor in determining volumetric breast density. It is also an important factor in determining dose calculation (for example, Wu. X., Gingold E. L. Barnes G. T., Tucker D m Normalized average glandular dose in Mo/Rh and Rh/Rh target-filter mammography radiology 1994 193 at 83 to 89; Law, J., editor: The Commissioning and Routine Testing of Mammographic X-ray System, Institute of Physical Sciences in Medicine, York UK, (1994) 59; Dance, D. R.: Monte Carlo Calculation of Conversion Factors for the Estimation of Mean Glandular Breast Dose, Phy Med Biol, 35, (1990) 1211-1219. The measurement of actual thickness provided by commercial mammography can be as much as one centimeter off the actual thickness due to deflection of the breast compression plate.
Accordingly, a mammography apparatus and method that improves the accuracy of measuring breast thickness is desirable.